Customers Shouldn't Pay for My Mistakes

The birthday shirts were a bust. I made the birthday boy's too big and the replacement did not make it in time for his Party. This is very discouraging. This has prompted me to approach my ordering system and improve it to avoid common mistakes that can make or break a customer's experience. I can't really make a custom order form as Etsy has it's own purchase system in place. I plan to include a Jerzees brand size chart and more explicit instructions with each of my T-shirt listings. Hopefully, I can begin setting myself up for success. My customers already pay in good faith for a quality product. They shouldn't have to pay more for my incompetence.